The main method of production of nitrogen and oxygen, two of the largest volume chemicals, is by cryogenic distillation of air. During the past three decades, air separation systems based on selective adsorption of nitrogen or oxygen have been commercialized, especially for smaller volume uses. A commonly used adsorption method for producing nitrogen from air is pressure swing adsorption (PSA). The feed to this process is typically air but may be nitrogen enriched air. Typically, it is a two bed process operating with a fairly simple cycle, that includes pressurization, product withdrawal at high pressure, pressure equalization of the beds and depressurization. Other steps such as a product purge may be used. The total cycle time is on the order of minutes. The high pressure is typically 4 to 8 atmospheres and depressurization is typically to atmospheric pressure though vacuum could be employed.
Presently, the preferred adsorbents for carrying out PSA air separations are either zeolites or carbon molecular sieves. Typically, zeolites are used when enriched oxygen (about 95%) is desired, and carbon molecular sieves are used when enriched nitrogen (about 99.5%) is the desired gas.
Carbon molecular sieves (CMS) are practical for separating oxygen from nitrogen because the rate of adsorption of oxygen is higher than that of nitrogen in these materials, i.e., a kinetic separation is effected. The difference in adsorption rates is due to the difference in size of the nitrogen and oxygen molecules; 0.364 nm and 0.346 nm, respectively. However, at equilibrium carbon molecular sieves do not have significant selectivity for either oxygen or nitrogen. Because the size difference of the molecules is so small, the CMS pore structure must be fine tuned in order to efficiently separate O2 and N2 on the basis of differing diffusion rates. U.S. Pat. No. 5,447,557 discloses an improved process for air separation using an oxygen selective adsorbent comprising a carbon molecular sieve impregnated with an acidic species.
When using zeolites, an equilibrium separation of oxygen and nitrogen is typically observed. This results from the difference in the quadropole moment between nitrogen (0.31 Å3) and oxygen (0.1 Å3). The nitrogen interacts more strongly with zeolitic cations than oxygen. Typically, for these processes vacuum swing adsorption (VSA) methods are used. Lithium and calcium have been disclosed as exhibiting particularly strong affinities for nitrogen (see U.S. Pat. No. 5,417,957 and U.S. Pat. No. 5,454,857).
PSA air separation processes make use of relatively simple equipment and provide easy maintenance when compared to cryogenic processes. Some drawbacks of PSA processes are lower product recovery and higher energy consumption than the cryogenic processes. Also, the high cost of currently available adsorbents and the product variability are major problems. Thus, there is a need for processes utilizing better adsorbents.